Magnesium cast alloys with little tendency to hot-crack

ABSTRACT

Magnesium cast alloys containing 3-10 percent aluminum, 0.3-2 percent zinc and 0.1-0.4 percent bismuth or tin, all by weight, showing little tendency to hot-crack.

0 Minted Mates Pmm [151 3,653,%@ Gitlesen 1 Apr. 4, 1972 54] MAGNESIUM CAST ALLOYS "WITH 1,942,041 1 1934 Wood ..75/168 B LITTLE TENDENCY 0 H0T CRAK 1,975,375 10/1934 Schichtel ....75/168 D 1,975,376 10/1934 Schichtel ....75/l68 D [72] Inventor: Gunnar Gitl n, R r gr nn, rw y 2,000,115 5/1935 Wood ....75/l68 C 2 041 866 5/1936 Schichtel ....75/l68 C N H N [73 Assgnee 2,041,867 5/1936 Schichtel ..75/168 [22] Filed: Jan. 8, 1970 Primary Examiner-Charles N. Lovell [2]] Appl' L449 AttorneyWenderoth,Lind&Ponack [52] US. Cl ..75/168 C, 75/168 R, 75/168 B, [57] ABSTRACT 148 32 51 lm. Cl ..C22c 23/00 Magnesium Cast allys maining Percent aluminum, 58 Field of Search ..75/168; 148/32 Percent Zinc and Percem bismuth by weight, showing little tendency to hot-crack. [56] References Cited 4 Claims, 1 Drawing Figure UNITED STATES PATENTS 2,094,332 9/1937 Schichtel et a1 ..75/168 C "C Better than "Standard" A) w 0 O Patented April 4, v 1972 GUNNAR GITLESEN,

INVENTOR lJMd ATTORNEY MAGNESIUM CAST ALLOYS WITH LITTLE TENDENCY TO HOT-CRACK The invention relates to an aluminium-containing magnesium cast alloy with little tendency to hot-crack.

Magnesium alloys containing 3 to percent of aluminum and usually 0.3 to 2 percent of zinc, for instance AZ 91, have several good properties when used for casting purposes, of which may be mentioned adequate. fluidity (Formfullvermogen), die castability and also satisfactory mechanical properties at temperatures up to about 100 C.

These alloys are not particularly liable to hot-crack, but their field of application would be even wider if the tendency to hot-crack could be further reduced, as a modern technique of pressure die casting provides possibilities for mass production of a growing variety of cast articles, including articles of such a design and shape as to increase the liability to hotcrack, thus requiring an alloy which per se has little tendency to hot-crack.

Hot-crack may occur in castings in the solid-liquid interface during the process of cooling.

The above-mentioned type of cast alloys have a considerable temperature range of solidification, and consequently stress conditions more easily arise in the casting during solidification, for instance due to hindered contraction occurring when the casting mould and/or the core prevent the free contraction of the casting, and also due to contraction distortion in the casting itself. The tendency to hot-crack is increased when the various portions of the casting are not cooled at the same rate, which will readily occur when the casting is not ofa fairly uniform thickness.

Different ways and means are known to counteract the tendency of cast alloys to hot-crack. As to magnesium alloys of the above-mentioned type, it is known that variation of the aluminum and zinc content has an influence on the alloys tendency to hot-crack.

Surprisingly it was found that adding bismuth to magnesium alloys of the above mentioned type has a very marked effect in reducing the hot-crack tendency of the alloy. Tin was also found to have a favorable effect, although not as strong an effect as bismuth.

The actual proportions of bismuth or tin used in practice are below 1 The experiments carried out show a maximum effect at 0.1 to 0.4 percent by weight of bismuth. The maximum effect of tin is also found to be between 0.1 to 0.4 percent by weight, although the effect in this case is, as mentioned above,

Z by weight of bismuth k by weight of tin These alloys were treated as to hot-crack in a modified per manent mould of the type described in Giesserei," 45, Heft 26 1958), 9761-765.

The results will appear from the graph. The ordinate represents the temperature deviation in degrees Centigrade from the critical temperature for AZ 91 without bismuth or tin. The term critical temperature as used herein means the lowest mould temperature that gives a crackfree test ingot.

The curve for bismuth shows a tem erature deviation of to C. for an addition of bismuth o 0.2 to 0.3 percent. This is a very great difference in temperature, which means that the favorable effect of an addition of 0.2 to 0.3 percent of bismuth to the alloy is considerable.

The graph further shows a similar effect with tin of 30 to 40 C. between 0.1 and 0.4 percent by weight of tin. The tin content may be within the range 0.2 to 0.3 percent by weight.

2. An addition of 0.3 percent of bismuth to an AZ 61 alloy (6 of aluminum, 0.7 percent of zinc, 0.2 percent manganese) gave an improvement of 55 C. compared with A2 61 without bismuth in experiments with the above-mentioned mould.

The alloys under 1) and 2) showed a tensile strength normal for A2 91 alloys. Further experiments showed that the addition of bismuth did not give increased tendency to corrosion as tested by dipping in a 3 percent NaCl solution.

B. AZ 91 alloys with and without bismuth were pressure die cast to a special hot-crack sensitive place. The tendency to hot-crack was examined in the cast pieces.

Result:

AZ 91: 4 cracks per piece (average for 200 pieces) 3 percent entirely faultless pieces AZ 91 with 0.3 percent Bi: 3.3 cracks per piece (average for pieces) 7 percent entirely faultless pieces It is evident from the above experiments that the hot-crack tendency in aluminium-containing magnesium alloys can be prevented to a large extent by addition of bismuth or tin, particularly bismuth.

The aluminum content of the alloys of this invention may be 3-10 percent by weight for tin-containing alloys and 5.5-10 percent by weight, including the narrower range of 7.5-l0 percent, for bismuthcontaining alloys.

1 claim:

1. Magnesium cast alloy consisting essentially of 3 to 10 percent by weight of aluminum, 0.3 to 2 percent by weight of zinc and 0.1 to 0.4 percent by weight of bismuth or tin, balance magnesium with the proviso that the aluminum content of the alloy containing bismuth is 5.5 to 10 percent by weight, said alloy having a reduced tendency to hot-crack.

2. The alloy according to claim 1, containing bismuth, wherein the aluminum content is from 7.5 to 10 percent by weight.

3. The alloy according to claim 1, wherein the content of bismuth or tin is 0.2 to 0.3 percent by weight.

4. The alloy according to claim 4, containing up to 0.5 percent by weight of manganese, up to 0.5 percent by weight of silicon and up to 0.5 percent by weight of copper. 

2. The alloy according to claim 1, containing bismuth, wherein the aluminum content is from 7.5 to 10 percent by weight.
 3. The alloy according to claim 1, wherein the content of bismuth or tin is 0.2 to 0.3 percent by weight.
 4. The alloy according to claim 4, containing up to 0.5 percent by weight of manganese, up to 0.5 percent by weight of silicon and up to 0.5 percent by weight of copper. 